The applicant's prior International Patent Application No. PCT/AU01/00805 (WO 02/04930 A1) discloses axially aligning a plasma torch with the magnetic field maximum of an applied microwave electromagnetic field for excitation of a plasma. This induces a hollow generally circular cylindrical shaped plasma due to the well known “skin effect”. A plasma of this shape is desirable because it is easier to inject a sample into its cooler core for heating. The skin depth of this plasma, which decreases at increased frequencies, is quite thin (for example, at 2455 MHz the skin depth of an argon plasma has been measured as about 1 mm) and thus use of a polyatomic plasma forming gas (which gives a greater skin depth) is disclosed to improve the heating into the core region of the plasma for heating sample within that region.
Further research by the applicant has revealed that improved results are obtained compared to the invention of PCT/AU01/00805 by energising the plasma by simultaneous application of an electric and a magnetic field oscillating at microwave frequency. The improved results flow from better heating into the core region of the plasma and thus better thermal coupling between the sample and the plasma leading to improved analytical performance (sensitivity).
The discussion herein of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date established by the present application.